1. Field of the Invention
The invention relates to ink jet printers and recorders, and more particularly to the control of droplet motions and flight paths in ink droplet printers.
2. Description of the Prior Art
In ink jet printers, liquid droplets are projected in a uniformly spaced relationship onto a moving print medium. The droplets are given an electrostatic charge during generation for subsequent electrical deflection by a pair of electrodes for controlling the droplet flight path for proper character printing.
Heretofore, droplet generation frequencies have been increased to such an extent that the effects due to air drag forces on the droplet moving through ambient air have become noticeable. One effect which is prominent in high speed droplet projection is an air retardation effect. This effect occurs because some droplet paths are longer than others during character printing, and therefore a relative time delay will occur by the air retarding those droplets traveling longer distances. During this time delay the document has moved a slight distance making the droplet's impact upon the printing medium fall downstream from its proper print location. Thus, air retardation gives a progressive vertical curvature to each vertical character stroke proportionate with the time delays of individual droplets.
A second effect of air drag is associated with a wake of disturbed air which follows directly behind each droplet. As the droplet's flight speed increases relative to the air, a wake turbulence forms behind each droplet disturbing the immediately following droplet. One effect of a droplet following in the turbulent area of the wake of a preceding droplet manifests itself as a "wandering" of the individual droplet about its proper point of impact on the print medium. A second effect occurs when the trailing droplet experiences less of an air drag force than the leading droplet, whereupon the trailing droplet eventually collides and merges with the leading droplet.
To reduce air disturbance effects on ink droplets, the prior art has suggested the use of a laminar flow of air collinear with the path of droplet flight. See for example U.S. Pat. No. 3,596,275 issued to R. G. Sweet on July 27, 1971. The prior art has used such an air flow with droplet frequencies generally maintained in a 100 kilocycle range with droplet flight velocities of 500-600 inches per second, but lately the frequency range has been extended to the 300 kilocycle range with droplet flight rates of 2000-2200 inches per second. Because air turbulent forces increase geometrically with droplet velocity, the air turbulent forces occurring in the 300 kilocycle range are 6 to 7 times as great as those occurring in the 100 kilocycle range.
At such high flight rates a more rapid column of air is required to be forced along the droplet path in order to eliminate droplet turbulence. But problems arise due to a more rapidly moving air flow. As the air column confronts the moving document air disturbance erupts along the document interface. Such airwave turbulence and other distorting air patterns affect the droplets as they enter the printing interface.
Further, air turbulence may also be generated from the rapidly moving air column setting up wave patterns off the housing or bounding walls where such walls exist enclosing the ink jet printer.
Thus, with an increase in droplet flight velocities the conventional laminar flow of air necessary to abate such air disturbances will only add further turbulent disturbances near the print medium which will affect the droplet flight path. Therefore special apparatus for maintaining and controlling a high speed laminar air flow in ink jet printers has come to be regarded as highly desirable.
It is accordingly an object of the present invention to substantially eliminate air disturbance effects on inflight ink droplets in an ink jet printing device.
It is another object of this invention to maintain a high speed laminar column of air moving collinear with the path of inflight ink droplets in an ink jet printing device.
It is a further object of this invention to remove air disturbance effects on inflight ink droplets at the printing interface where a laminar column of air confronts the print medium.